(a) Field of the Invention
The present invention relates a laminate, and more specifically, it relates to an anisotropy-free laminate having high strength and high stiffness which comprises a specific orientated polyethylene layer and a specific adhesive layer.
(b) Description of the Prior Art
The so-called ultra-high-molecular-weight polyethylenes having noticeably high molecular weights are excellent in impact resistance and wear resistance and have self-lublicating properties, and therefore they are used as characteristic engineering plastics in many fields.
This ultra-high-molecular-weight polyethylene has a much higher molecular weight than a usual polyethylene, and thus it is known that a fiber or sheet having higher strength and higher stiffness than before can be obtained by highly orientating the ultra-high-molecular-weight polyethylene.
However, in the case of the sheet, physical values such as strength in a direction perpendicular to an orientating direction and modulus of elasticity are extremely low owing to its high anisotropy, and thus applications of the sheet are limited. Here, it can be expected that the directional property-free sheet having high strength and high stiffness can be obtained by superposing and sticking, for example, the two sheets on each other, these sheets being mutually diverted from each other as much as an angle of 90.degree., but when the conventional adhesion technique is employed, adhesive force is weak and physical properties such as strength and stiffness deteriorate inconveniently. For this reason, such a technique is not practical.
For example, in a suggested laminate in which the ultra-high-molecular-weight polyethylene is used, a press molded rod-like or plate-like ultra-high-molecular-weight polyethylene material is skived to form a sheet, and the latter is then stuck on another substrate via an adhesive polymer (Japanese Patent Laid-open Publication No. 143137/1986).
However, this kind of laminate is proved with a feature of the ultra-high-molecular-weight polyethylene such as wear resistance, but it has some drawbacks of the other kind of material simultaneously. In addition, functional effects such as high strength and high stiffness cannot be obtained because of non-orientation. Therefore, the suggested laminate is not always satisfactory from the viewpoint of performance.
On the other hand, in order to form the laminate, it is necessary to heat the resins of both the layers up to a higher temperature than their melting points so as to thermally fuse them, but When the orientated materials are heated in excess of their melting points, the effect of orientation is lost and tensile performance deteriorates noticeably.